As injectors (fuel injection valves) for injecting fuel into cylinders of an internal combustion engine mounted on a vehicle, an electromagnetic solenoid-type injector is used. This injector is driven to open its valve when its coil is powered by electric current. A fuel injection control apparatus, which controls fuel injection to the internal combustion engine by driving the injector, controls a fuel injection time and a fuel injection quantity by controlling a drive start time and a drive period. The drive start time is a time point of starting a power supply operation for supplying current to a coil. The drive period is a time interval for continuing the power supply operation from the drive start time.
It is proposed in this kind of fuel injection control apparatus to detect a characteristic of the injector and correct the drive period or the like of the injector in accordance with the detected value (for example, in accordance with a quantity of deviation of the detected value from a reference value).
As one technology for detecting the characteristic of an injector, JP 2010-532448A, for example, discloses calculation of a period (closing period), which starts from a start time of a valve closing process to a valve closing time as the injector characteristic. The valve closing time of the injector is detected based on a differential value calculated by differentiating a current of the coil, which decreases from the start time of the closing process (corresponding to an end time of the drive period) of the electromagnetic valve corresponding to the injector. Further, JP 2010-532448A discloses that, for realizing a required fuel injection quantity, the drive control continuation period (corresponding to the drive period) is calculated by using the calculated valve-closing time.
According to the fuel injection control apparatus disclosed in JP 2010-532448A, the differential value of the current of the coil (coil current) is calculated by analog/digital-converting the coil current by an analog/digital converter (ADC) at every predetermined interval and differentiating each A/D-converted value.
This calculation increases processing load and needs an A/D conversion channel of the A/D converter in the fuel injection control apparatus.